Indirectly heated cathode



March 30, 1937. L EWE ET AL 2,075,122

INDIRECTLY HEATED CATHODE Fil ed May 26, 1931 Jnren/or v @MJk- 22:2 WW4 Patented Mar. 30, 1937 UNITED STATES PAT]:

-OFFlCE Siegmund Loewe and Bruno Wienecke, Berlin, Germany Application May 26, 1931, Serial No. 540,185 lin Germany May 28, 1930 4 Claims.

The invention relates to indirectly heated cathodes, the highly emissive surface of which consists of barium or a similar highly emissive metal.

Cathodes of this type should possess the following characteristics which, until now, could not all be secured in one device.

1. The emissive layer must adhere to the surface of the cathode in such a manner that it does not split away during the operation of the tube.

2. The cathode must possess a perfect equipotential area of good conductivity.

3. The equipotential area must be sufficiently insulated from the points of the filament which have difierent potentials.

4. The cathode must possess suificient heat inertia so that the inavoidable small fluctuations in temperature of the filament will not cause audible fluctuations of the emission current.

5. The time required for heating the cathode should not be excessive.

6. No destructive stresses (mechanical pressure, tension) should result from the fiuctuations in temperature of the cathode.

7. All electrical contacts must remain perfect at the temperatures concerned.

8. The construction of the cathode and its holders must be so strong that mechanical movements of the cathode with relation to the other parts of the system cannot occur, even when the entire valve is moved or shaken.

9. The remnant gases should be bound during the step of vaporizing the metallic barium on to the cathode surface.

The object of the invention is an indirectly heated cathode which is constructed in such a way as to fulfil simultaneously all of the stated conditions.

According to the invention, the insulating tube which may consist for example of porcelain, or fired mixtures of magnesia, silicates and the like is furnished, prior to the application of the barium, with a so tightly fitting metallic jacket that no portion of the barium, or at least no essential proportion thereof, impinges the insulating material when the barium is vaporized on to the cathode. Said jacket may consist of a massive cylinder or a piece of sheet metal disposed round the tube, or also a closely wound wire spiral. In those cases in which the jackets possess small gaps through which parts of the insulator are exposed, these gaps, slits or the like are preferably so disposed that the main quantity of the barium vaporized on to the oathode is withheld therefrom. If a reaction mixture is employed forproducing the barium, and a slotted sheet metal jacket is employed, the side of the jacket remote from the slot is disposed opposite the reaction mixture.

The metal jacket is then coated with that 5 particular metal on which the barium efficiently adheres and provides good emission for example tungsten. Said tungsten may be applied in the form of a tungsten wire wound upon the metal jacket. It may also be applied by means of 10 spraying or by reduction of tungsten hexachloride.

According to the invention, the metallic base for the tungsten is made of a material which possesses a greater coefiicient of expansion than 5 tungsten or the particular metal which is applied to the metallic jacket. When employing tungsten wire as a carrier for the barium, there is preferably chosen a metallic cylinder composed of nickel. This, upon heatingyexpands 20 to a greater extent than tungsten; the greater, therefore, the extent of heating, the more the tungsten wire presses against the nickel jacket. Thus shaky contacts or disturbing humming noises cannot occur. The tungsten wire may 25 under certain circumstances fit so tightly against the nickel jacket that the same later, even after cooling, constitutes a unitary whole. In any case, however, the hitherto existing difficulty is overcome. Even should the insulating tube now possess a considerably smaller coefl'icient of expansion thanthe nickel jacket, the latter nevertheless forms such a solid unit that even with a certain looseness in the connection between porcelain tube and nickel jacket contact troubles or periodic contact or current fluctuations do not occur in the heated state. v

According to a further object of the invention, insulating tubes are employediwhich, in propor-' tion to their diameter, possess only a relatively 40 small thickness (for example, .3 mm. thickness of wall with tubes .8 mm. thick). The use of insulating tubes of this kind having a comparatively great inner diameter allows of using a spiralized filament instead of the straight filament 45 which otherwise must be employed. Since the emissive layer requires to be raised merely to a low temperature, the heating period, in accordance with requirement 5, is small, particularly as the heat capacity of the porcelain tube is consid- 50 erably less owing to the reduced thickness. On the other hand, due to the low emission temperature, the cooling of the cathode is considerably less than in the case of higher temperatures. The .1955 Of heat at those times when the filament is 55 traversed to a less extent, or not at all, by the current (i. e., in the case of A. C. heating at the times of the nodal points of the heating potential) is accordingly less than heretofore, and the heat inertia of the cathode therefore greater.

Several embodiments of the invention are illustrated by the accompanying drawing in which Fig. 1 shows an electrondischarge-device .conv taining the indirectly heated cathode according to the invention, Fig. 2 shows a longitudinal 'sect'ion, Fig. 3 a cross-section of the cathode'of the;

' diameter of which may amount to between three and five times the thickness of the wall, is traversed by a single-thread spiral filament 2, of.

tungsten wire which, must beraised to a temperature-'of more than 1000" 01100-1400 The porcelain tube is surrounded by a nickel jacket 3 which, in the-manner'shownsectionally in Fig. 3, may be produced from a folded piece of sheet metal. To this nickel jacket there is. applied a layer'of tungsten wire, for example wound. in the form ofa helix 4?. The nickel base of this tungs sten wirepossesses at5: the tapping contact which establishes an efficient. conductive connection with the single wire: windings .or other tungsten sections, and over the latter with the layer of barium 5 which is produced on the tungsten wire. If barium.- is vaporized on to the cathode in the manner illustrateddiagrammatically by arrows in. Fig. 1, not only the tungsten wire; is furnished with a 'thin coating ofi the metal, which later acts as emissivelayer-,but also the nickel is coveredthereby. This barium deposited on the nickel serves as..a. gas binding. agent: during the operationof theftube. 1 7

By the. use of a slotted piece of sheet metal it .-.is possible to secure the additional; advantage that thenickel jacket fitstightly against; the

porcelain tube in the warm state, as the slot in the nickel jacket contracts upon heating without limiting the degree of contact with the metaltube.

This efiect maybe amplified. by the, counter-pressure exert-ed'by a. material havinga smaller ccefiicient of. expansion. such as tungsten. Alternatively, a favourable; di tribution of pressure is capable of being attained with the assistance, of

a. nickel J'ackeh. which composed. f; windin s ohn-icke1 wireiFig; 4). Bar oumrlydesirawe is an: embodiment acc rdin to- (crosse tion), in which the two ed es of; the'she t nicke overlap; 1

It has furthermore .beenpfbund particu arl suitable to. apnlyai-nick -jbase i his kind y means of. the-sp ay pro es he n ck zbasema also be produced ele tr yt cally the n w Figs. 4 and 5 illustrate in equalman'ner,

. of the nickel, the additional advantage that very good. contact is produced between the equipotential cathode and the cathode tapping. Particular advantages are involved if the layer of nickel is created by separation from a gaseous nickel compound, for example from a 'nickel-tetracarbonyl atmosphere. This is particularly suitable in systems. with very weak heating power, which in consequence have only a very small thickness of porcelain tube and may therefore be very easily strained mechanically by the spraying process.

We claim:

1., Anindirectly heated electron emitting cathode for an electron discharge device comprising a heater filament, an insulating tube surrounding said heater filament, a metal layer surrounding said insulating tube, a further metal layer the coefficient of expansion of which is lower than that of the first metal layer surroundi ing the first metal layer and consisting of a spiral wire, and a highly emissive layer surrounding the r spiral wire.

2. An indirectly heated electron emittingcathode for an electron discharge device comprising a heater filament, an insulating tube surrounding said heater filament, a metal layer surrounding said insulating tube, a further metal layer the coeflicient of expansion of which is lower than that of the first metal layer surrounding the'first metal layer and consisting of'a helix of tungsten wire, and a highly emissive layer surrounding said helix of tungstenwire.

3. An indirectly heated electron emitting cathode for an. electron discharge device comprising a heater filament, an insulating tube surrounding said heater filament, a metal layer surrounding said insulating tube, afurther metal layer the coefiicient of expiansion 0,1 which is lower than that of the first metal layer surrounding the first a highly emissive layer 01' barium surroundingv the spi al wire 4. An indirectly heated electron; emitting cathod or an ele tron. dischar e device. compr sin a heater filament, aninsulating tube surroundn saidheate fi ament. a metal a er su roundn said ns la ing ubaa u h r metal layer the coefiicient of expansion of which islower than t a fth fir metal la e u ou diristhe first metal layer and consisting of a spiral wire, and 

